Support persistent IPv6 address lookup

[unlimitedsola]

Currently, the IPv6 address is retrieved by making requests to external services which echo back the outgoing IP address that is used to make the request. This approach was chosen due to its ease of implementation across all platforms.
However, with IPv6 Privacy Extension enabled, the client will use a temporary address to make the request and the address will be changed from time to time. Even though the address will be routable for a short period of time, it is not ideal nor reliable to be used for DDNS purposes.
It would be a nice addition if we can find a reliable way to retrieve the persistent IPv6 address across major platforms and have the ability to configure it specifically for IPv6.